Mortal Instruments City of Bones- Why did the author write this type of work?

I think the author wrote this book for two reasons. One is because it is for entertainment. It is action-packed and full of cool suprises. It is really intresting and it is very unrealisitc, which makes you enjoy it even more. The book inculeds fantasy type powers, and cool tools that they use. Another reason is for the theme. I think that out of every book there is at least one meaningful theme to the story. Even though I haven't finished the book yet, I'm sure I will come across one. Especially at the end of the book, where the story wraps itself up. Also it is where most of the characters apoligize, and get back together (if they had become seperated or mad at each other).